A silylene-stabilized ditin(0) complex and its conversion to methylditin cation and distannavinylidene

Due to their intrinsic high reactivity, isolation of tin(0) complexes remains challenging. Herein, we report the synthesis of a silylene-stabilized ditin(0) complex (2) by reduction of a silylene-supported dibromostannylene (1) with 1 equivalent of magnesium (I) dimer in toluene. The structure of 2 was established by single crystal X-ray diffraction analysis. Density Functional Theory calculations revealed that complex 2 bears a Sn=Sn double bond and one lone pair of electrons on each of the Sn(0) atoms. Remarkably, complex 2 is readily methylated to give a mixed-valent methylditin cation (4), which undergoes topomerization in solution though a reversible 1,2-Me migration along a Sn=Sn bond. Computational studies showed that the three-coordinate Sn atom in 4 is the dominant electrophilic center, and allows for facile reaction with KHBBus3 furnishing an unprecedented N-heterocyclic silylenes-stabilized distannavinylidene (5). The synthesis of 2, 4 and 5 demonstrates the exceptional ability of N-heterocyclic silylenes to stabilize low valent tin complexes.


Reviewers' Comments:
Reviewer #1: Remarks to the Author: The manuscript by Mo and coworker describe the synthesis of a ditin compound substituted with twπo N-heterocyclic silylenes (cpd 2).This compound is isoelectronic to a diphosphene with P=P double bond.Consequently, the bonding analysis of compound 2 reveals a Sn=Sn double bond with a lone pair at each Sn atom.The methylated species 4 has no resemblance to an alkynyl cation, the methylation occurs at the lone pair of the tin atom which is orthogonal to the π-system.The reaction of a hydride source with 4 leaves the multiple bond unaffected, very unusual for an distannynyl cation.The hydride adds to the NHSi indicating that the polarization of the NHSi-Sn(1)-bond in compound 4 (and 2) is (+)NHSi-Sn(-) and no arrow is needed.In summary, the synthesized compounds are interesting and well characterized.The computations are state of the art; however, I do not agree with the interpretation of the data.For the interpretation of the data, I recommend to compare the synthesized compounds with the isolobal phosphorous species and not with group 14 compounds.In summary, the data provided is fine, the interpretation is misleading and in the present version I do not recommend publication of this manuscript.There are other mistakes that should be erased during revision of the manuscript.Line 10.The structure of 2 was established by single crystal X-ray diffraction analysis (not be spectroscopic analysis).Line 16. Are these compounds indeed "highly reactive".The reagents used are MeOTf and KHBBu3 which are highly reactive reagents.Line 26.The NHC stabilized diatomic main group compounds are not allotropes of these main group elements Figure 1b.Please specify R3Si.Line 36.These compounds are not allotropes of the elements.Line 44.Please use plural (….silylenes ….analogues…) Line 47.What is meant with a thermodynamic target?Please rephrase.Line 53.Please define NHSi as LSiNHI Lines 64-67.The involvement of NHSi coordinated Sn2H2 in the formation of 2 is a speculation, please omit.NMR characterization of compound 2. Tin NMR data is missing, the SI informs that this is due to the high anisotropy of the chemical shift tensor.This might be true, however, give the spectral range which was investigated, either in the main text or in the SI material.Line 85. …"(2.7225(5)Å),6 but" please check or erase the "6" Lines 85/86 actually the reported SnSn distance is longer than the quoted example.Line 86: The compound should have a triple bond between the tin atoms Line 87. 40 is the wrong reference.Lines 92-93.The statement " The optimized structure of 2 is in excellent agreement …. " is completely unsupported.What is excellent (1% or 5% deviation)?Please provide a comparison of important computed and experimental structural data (SI material).Lines 118-119: I do not agree that complex 4 with the composition [(NHSi)(Me)SnSn(NHSi)]+ (one Sn tricoordinated, the other dicoordinated) is an analogue of R-CC+ (one carbon dicoordinated the second monocoordinated).Please rephrase.Line 129. …methonium….. Lines 143-152: Please give the WBI of the SnSn bond in compound 4 for comparison.Lines 185 -187.Sn1 has a trigonal planer coordination environment and the coordination geometry of Sn2 is V-shaped.Please rephrase.Lines 196 The HOMO of 5 (not of 2) References 38 and 40 are identical.

Reviewer #2:
Remarks to the Author: Mo and co-workers present a manuscript about silylene stabilized ditin compounds.The presented chemistry is certainly of interest to the community of main group element chemists.This referee doubts that this ditin chemistry is of high interest to the broad readership of Nature communications and therefore suggests publication in a more suitable journal.
The missing 119Sn NMR signal of 2 was only commented in the experimental section.This comment is missing in the manuscript.The missing 119Sn NMR signal of 4 was also only commented in the experimental section.Low temperature 119Sn NMR measurements are needed.Because of the importance of the tin atoms in this chemistry more spectroscopic tin data are necessary.To this referee it is very uncommon not to comment on the missing 119Sn NMR data in a manuscript on tin chemistry.The name distannynyl cation of 4 was derived from the term carbyne cation.To this referee the name distannynyl cation (4) suggests a triple bond could be discussed in the compound so designated.However, the cation 4 consist of a Sn=Sn double bond with one tin atom alkylated and coordinated by a silylene, therefore the name distannynyl cation should not be used.The hydride addition to compound 4 is interesting and the position of electrophilicity in cation 4 belongs to the NCN-ligand connected at the silicon.The authors did not comment on this reaction.A discussion should be added.What about the LUMO of cation 4? Citations: line 33, citation 33 is in wrong position.line 40, citation 6 is wrong at this position, it should be 11, the Sn2-Jones paper typos: line 85, … 2.7225(5) Å),6 but… line 86, ..shorter than… should be longer than.119Sn NMR data should not be listed with two decimals.

General Comments:
The manuscript by Mo and coworker describe the synthesis of a ditin compound substituted with two N-heterocyclic silylenes (cpd 2).This compound is isoelectronic to a diphosphene with P=P double bond.Consequently, the bonding analysis of compound 2 reveals a Sn=Sn double bond with a lone pair at each Sn atom.The methylated species 4 has no resemblance to an alkynyl cation, the methylation occurs at the lone pair of the tin atom which is orthogonal to the π-system.The reaction of a hydride source with 4 leaves the multiple bond unaffected, very unusual for a distannynyl cation.The hydride adds to the NHSi indicating that the polarization of the NHSi-Sn(1)-bond in compound 4 (and 2) is (+)NHSi-Sn(-) and no arrow is needed.In summary, the synthesized compounds are interesting and well characterized.The computations are state of the art; however, I do not agree with the interpretation of the data.For the interpretation of the data, I recommend to compare the synthesized compounds with the isolobal phosphorous species and not with group 14 compounds.In summary, the data provided is fine, the interpretation is misleading and in the present version I do not recommend publication of this manuscript.There are other mistakes that should be erased during revision of the manuscript.

Response:
We thank the reviewer for the high evaluation of the quality of this work and for the comments given below.In the light of the reviewers' comments, we have thoroughly revised the manuscript to improve this manuscript.According to the reviewer's suggestions, we have reformulated the tin compounds and modified the interpretation of the data.Compounds 2 and 4 are described as a ditin compound [{L(NHI)Si}Sn=Sn{Si(NHI)L}] and a NHSi-stabilized mixed-valent methylditin cation [{L(NHI)Si}Sn(Me)=Sn{Si(NHI)L}][B(Ar F )4], respectively.The structural comparisons between compounds 4 and 5 with the corresponding isolobal multiple-bonded phosphorous compounds are included in the main text.The main comparisons are summarized as following: "The methylditin cation 4 and distannavinylidene 5 are isolobal to multiple-bonded phosphorous compounds such as diphosphanyl cations (Fig. 1, A) and stannaphosphenes (Fig. 1, B).These structures display a trigonal planar geometry at the three-coordinated Sn and P atom.In addition, the ability of the ditin(0) complex to transfer the Sn(0) to other substrates was investigated.The reactions of complex 2 with DippN3 and iminobenzoquinone afforded the tetrameric tin imido complex [(SnNDipp)4] and bis(amidophenolato)tin complex, highlighting its synthetic utility.
Comment 1: Line 10.The structure of 2 was established by single crystal X-ray diffraction analysis (not be spectroscopic analysis).

Response:
We thank the reviewer for the comment.The description of spectroscopic analysis has been changed to "single crystal X-ray diffraction analysis" according to the reviewer's suggestions.
Comment 2: Line 16. Are these compounds indeed "highly reactive".The reagents used are MeOTf and KHBBu3 which are highly reactive reagents。

Response:
We thank the reviewer for the comment.We have modified the description.

Comment 3:
Line 26.The NHC stabilized diatomic main group compounds are not allotropes of these main group elements.

Response:
We thank the reviewer for the comment.The description of "serve as soluble allotropes" has been changed to "serve as precursors".

Response:
We thank the reviewer for the comment.We have redrawn the structures in the Fig. 1b Comment 5: Line 36.These compounds are not allotropes of the elements.

Response:
We thank the reviewer for the comment.The description of "diatomic allotropes" has been changed to "zero-valent diatomic main-group compounds".

Response:
We thank the reviewer for the careful examination.The "silylene" and "analogue" have been changed to "silylenes" and "analogues" according to the reviewer's suggestions.

Comment 7:
Line 47.What is meant with a thermodynamic target?Please rephrase.

Response:
We thank the reviewer for the comment.We apologize for the inaccurate expression.The " thermodynamic and synthetic targets" has been changed to "synthetically achievable" according to the reviewer's suggestions.

Response:
We thank the reviewer for the comment.The "NHSi" has been changed to "LSiNHI" according to the reviewer's suggestions.

Comment 9:
Lines 64-67.The involvement of NHSi coordinated Sn2H2 in the formation of 2 is a speculation, please omit.

Response:
We thank the reviewer for the comment.We have omitted the description about NHSi-coordinated Sn2H2 intermediate according to the reviewer's suggestions.
Comment 10: NMR characterization of compound 2. Tin NMR data is missing, the SI informs that this is due to the high anisotropy of the chemical shift tensor.This might be true, however, give the spectral range which was investigated, either in the main text or in the SI material.

Response:
We thank the reviewer for the high evaluation of the quality of this work and for the comments given below.In the light of the reviewers' comments, we have thoroughly revised the manuscript to improve this manuscript.We have further studied the ability of the ditin(0) complex to transfer the Sn(0) to other substrates.The reactions of complex 2 with DippN3 and iminobenzoquinone afforded the tetrameric tin imido complex [(SnNDipp)4] and bis(amidophenolato)tin complex, highlighting its synthetic utility.These new results have been included in the revised manuscript.
The synthesis of molecules that featuring main-group elements in unusual oxidation states is a primary pursuit of main-group chemistry, which expands our understanding of bonding and electronic structure and serves as the foundation for the construction of long sought after molecules that would otherwise remain inaccessible.In this contribution we demonstrate the synthesis, structural characterization and reactivity of a silylene-stabilized ditin(0) complex.
The key features of this work are briefly summarized below: 1) Synthesis and characterization of a silylene-stabilized ditin(0) complex.The chemistry of zero-valent main-group element complexes emerged recently and has attracted increasing interest due to their unusual electronic structures and their synthetic potential.Due to their low electronegativity and the less effective pπ-pπ orbital overlap of tin atom, isolation of tin(0) complexes remains a great challenge.We demonstrates the utility of a strongly electron-donating N-heterocyclic imino substituted silylene in stabilization of highly reactive ditin(0) complexes, providing a new strategy to probe the exciting chemistry of highly reactive tin(0) complexes.In terms of utility, the ditin(0) complex is capable of transferring the Sn(0) fragment to other substrates.
2) Developing a strategy to access unprecedented ditin cation via alkylation of the ditin(0) complex.Ditin cation are intriguing targets as they possess two highly reactive functional groups including a main-group element cation and a multiple bond.However, the synthesis of such species is extremely challenging as the lack of a general method.Addition of a small electrophile (Me + ) to the silylene-stabilized ditin(0) afforded a methylditin cation featuring mixed-valent Sn(II) and Sn(0) centers, which undergoes topomerization in solution though a reversible 1,2-Me migration along a Sn=Sn bond.
3) Synthesis and characterization of the first distannavinylidene.According to the computational findings, the positive charge in ditin cation is mainly at the three-coordinate Sn atom.Thus, the addition of nucleophiles to it might give the tin analogues of vinylidene, which remains exclusive in spite of considerable efforts have been made to synthesize heavier analogues of vinylidene compounds over the past decade.The ditin cation reacts with KBH(sec-Bu)3, where hydride attacks the amidinate ring to form the an NHSi-stabilized distannavinylidene, providing another example for the exceptional ability of NHSis to stabilize low-valent tin centers in unusual bonding environments.
As such, we strongly believe this paper will be of broad general interest, not only to those working in the highly topical field of main group chemistry, but also to colleagues working in the broader fields of synthetic chemistry.

Comment 1:
The missing 119 Sn NMR signal of 2 was only commented in the experimental section.This comment is missing in the manuscript.

Response:
We thank the reviewer for the comment.We apologize for not commenting the missing 119 Sn NMR signal of 2 in the manuscript and the corresponding description has been added.

Comment 2:
The missing 119 Sn NMR signal of 4 was also only commented in the experimental section.Low temperature 119 Sn NMR measurements are needed.Because of the importance of the tin atoms in this chemistry more spectroscopic tin data are necessary.

Response:
We thank the reviewer for the comment.The low temperature 119 Sn NMR measurements of 4 has been performed according to your suggestion.The related data has been summarized in the Fig. 5 of SI Material and discussed in the main text."The 119 Sn NMR spectrum of 4 recorded at 213 K shows two signals at δ 633.4 and 297.9 ppm, which correspond to the triicoordinate Sn(II) atom and the terminal two-coordinated Sn(0) atom, respectively.The resonance for the Sn(0) atom is shifted downfield relative to that of the bis(NHSi)-stabilized zero-valent tin complex (δ = -1147.2ppm)".